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Electrical Resistivity

Electrical resistivity is an intrinsic physical property of any material, i.e., how strongly the material can resist electric current, which ranges over many orders of magnitude. This characteristic of the subsurface can be measured as a function of the distance between points and used to map the subsurface resistivity structure.

Electrical Resistivity Testing (ERT), Electrical Resistivity Imaging (ERI) or Tomography and Marine Electrical Resistivity Imaging (MERI): This simplified example is based on a Wenner array: Current (I) is induced between paired electrodes (C1, C2). The potential difference (ΔV) between paired voltmeter electrodes P1 and P2 is measured. Apparent resistivity (Δa) is then calculated (based on I, ΔV, electrode spacings). If the current electrode spacing is expanded about a central location, a resistivity–depth sounding can be generated. If the array is expanded and moved along the surface, 2-D or 3-D resistivity–depth models can be created.

Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.

Electrical Resistivity Testing (ERT) or Vertical Electrical Sounding (VES) consists of 4 electrodes firmly hammered into the ground and connected vie electrical cables to the main console. The electrodes and cables are moved at pre-determined spacings for each reading.

Electrical Resistivity Imaging (ERI) or Electrical Resistivity Tomography (ERT) consist of several electrodes (e.g., 64, 72, 84, 96 etc.) firmly hammered into the ground and connected via 4 electrical cables to the main console. Normally, 1 or 2 cables at the beginning of the layout and respective electrodes are moved for each reading, i.e. “roll-along”. This generates an overlap of data with each reading.

Marine Electrical Resistivity Imaging (MERI) consist of several electrodes towed from a vessel that are connected to the main console on board vessel.

Figure 1: Sample of Electrical Resistivity Testing (ERT) tabulated data (extract from ERT report)

Figure 2: Sample of Electrical Resistivity Imaging (ERI)

Figure 3: Sample of Marine Electrical Resistivity Imaging (MERI)

What are the most common uses of Electrical Resistivity?

Geological structures/features detection/delineation, e.g., voids/cavity, boulder floaters, faulting and fault zone, dykes, palaeochannels
Subsurface electrical resistivities distribution for earthing/grounding system
Landfill extent delineation and quantification
Aquifer/aquitard delineation
Contaminant plumes detection and delineation
Mine working location

Electrical Resistivity FAQs

What is an Electrical Resistivity Testing/Imaging/Tomography?

Electrical resistivity survey is the measurement of the “ability” of material to resist the passing of electrical current.

As a known current is injected into the ground via current electrodes, the different material will modify the current flow based on the material property. This alteration will affect the electric potential distribution, which can be measured by potential electrodes at the surface.

How many types of Electrical Resistivity arrays?

The most common ER arrays are:

  • Wenner
  • Schlumberger
  • Wenner-Schlumberger
  • Gradient
  • Dipole-Dipole
  • Pole-Dipole
  • Pole-Pole

The selection of the type of array utilised depend on the required scope of the project, e.g., earthing system or geological linear structures detection at depth?

How much does an Electrical Resistivity survey cost?

As the applications for Electrical Resistivity are several, as well as the type of arrays, it is difficult, and misleading, to provide an exact dollar value. A very indicative cost for the fieldwork component can vary from $2,000 to $5,500 per day. This excludes mobilisation and demobilisation, which can vary by 10s of thousands of dollars, e.g., urban Sydney or Kalumburu in the Kimberly? Also, it excludes processing and reporting, which depends on the amount of data acquired.

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No matter how simple or complex is your project, we can assist in many ways…a technical explanation? A practical field implementation? A budget discussion? Or a full project planning and execution? Just ask away.